SOLE-LEATHER.--Treatment in the Tan-house.
On first coming into the yard, the butts are usually suspended by
the shoulder or butt ends from sticks placed across the pits. They
should be kept in almost constant movement, either by raising and
shaking them by hand, or by supporting them on frames, which are
rocked, or otherwise worked. Perhaps the best device for this purpose
is the "travelling handler" of W. N. Evans, which consists of a frame
supported on wheels, and worked slowly backwards and forwards by power.
This frame should extend the length of a range of pits sufficient to
take in at least a 3 days' stock of butts, which should be tied to
sticks resting crossways upon it. It should have a stroke of 1-2 ft.,
repeated, say 6 times a minute. The power required is very small.
The American rocker consists of a wooden frame balanced on its centre,
and made to oscillate by power. It is a cheap and efficient machine,
its defects being that the butts at the ends are much more moved than
those in the centre, and that their upper parts, being lifted out of
the liquor, are liable to become blackened.
The suspender pits should be supplied with old handler liquors, which,
if the tannage is a mixed one, may range from 12° to 20° barkometer,
as a large proportion of the weight consists only of lime-salts,
gallic acid, and other worthless products. It must here be explained
that the barkometer (also called "barkrometer" or "barktrometer") is a
hydrometer, graduated to show the sp. gr. thus--20° Bark. = 1·020 sp.
gr. In using it the temperature of the liquor must be at or near 60° F.
(15° C.). It is, of course, affected by any other matters in solution,
precisely the same as by tannins. In the Lowlights Tannery the waste
liquors are constantly about 12° Bark., and contain tannin equal to
less than 0·2 per cent. (expressed as crystal oxalic acid), and gallic
acid and similar matters equal to O·6 to 0·7 per cent. If the tannage
is pure bark, it may perhaps be advisable to let the strength be
somewhat less, but something depends on whether the exhausted liquors
are returned with all their impurities to the "taps" or liquor-brewing
pits, or whether the liquors are made with water, and hence purer. In
any case, the free acid in the suspenders should always be sufficient
in quantity to neutralise the lime brought in by the butts, or bad
colour will certainly result, making itself visible in the shed, or as
the tanning proceeds. If the butts, when first brought into liquor,
take a lemon-yellow colour, especially in places that have been
imperfectly exposed to it, this is an indication of danger which must
not be disregarded. It may be met either by cleansing the butts more
thoroughly before bringing into the yard, or by adding acid (acetic,
hydrochloric, or sulphuric) to the liquor. If this be done, great care
must be taken not to over-do it, and an acid free from iron must be
used. The use of sulphurous acid for the purpose has been patented, and
presents some advantages. Sulphites have been observed by the writer
to give a pink or purple reaction even with very dilute infusions of
valonia (see p. 112); but any coloration from this cause would probably
disappear as the tannage proceeds. The difficulty can, however, often
be remedied, either by altering the way of working the liquors, so
as to bring more sour liquor down to the suspenders, or by using a
larger proportion of materials capable of yielding acetic acid by
fermentation, such as myrobalans. It is a common error to call all the
free acid of sour liquors "gallic," as this is scarcely present in pure
bark-yards, and at the best is a very feeble acid. The most abundant
acid is usually acetic, though butyric, lactic, and other acids are
frequently present in varying proportions, according to the tanning
materials employed. In the English process, with its comparatively
short layers, in which the butts almost float in strong liquors, but
little souring takes place, and we have nothing comparable to the
German "sour bark" and "sour liquor" from long layers with weak liquor,
and much dusty material. These contain large quantities of acetic
and lactic acids, and plump almost like vitriol. Though the American
tanners generally use the latter, their hemlock liquors sour much
more intensely than those of English yards. It must always be borne
in mind, in comparing English with American and Continental tanning,
that, in the first, the opening up of the fibre is effected by lime,
and the swelling is maintained in the liquors, not so much by acids,
which are only present in very small proportion, as by the careful and
gradual working forward into infusions stronger and stronger in tannin;
while in the two latter, lime, if used at all, is simply employed to
loosen the hair, and the swelling and differentiation of the fibre
is first accomplished in the liquors either by vegetable or mineral
acids. Hence good results cannot be expected in English yards from such
processes as sweating or painting with sodium sulphide, which does not
plump, without a radical modification of the whole tanning process.
This point has been ably treated by Eitner in a series of papers
on _Extract-gerberei_, published during the last few years in 'Der
Gerber,' which will well repay attentive perusal by English as well as
German tanners.
The butts should at first be brought into the weakest liquor; a
circulation system, by which the liquors are all pumped in at one end
of a set of suspenders, and run out at the other, the butts being moved
forward in the opposite direction, seems to have much to recommend it.
In this case, the top of one pit should be connected by a wooden box
with the bottom of the next.
It is usually advisable to run away the first liquor into which butts
are brought from the lime-yard, as it is very completely spent, and
highly charged with lime salts and impurities. Whether other exhausted
liquors are to be retained or rejected is largely a question of
climate, and mode of working. In hot weather, such liquors, charged
with organised ferments (moulds, _bacilli_, and _bacteria_), are apt
to cause ropiness, and other fermentive diseases of the liquors. This
danger may be lessened by boiling all spent liquors, so as to kill the
ferments, before running on the taps, or prevented by the free use
of antiseptics, such as carbolic acid. Small doses of carbolic acid,
however, are useless; at least 1/10 per cent. must be employed; and it
must be borne in mind that antiseptics prevent souring as well as other
fermentations, and hence, where they are employed, other means must be
adopted to maintain the necessary acidity. Such liquors are very liable
to darken if boiled.
The suspender liquors should be acid enough freely to redden
litmus-paper. The present author has published a simple volumetric
method for the determination of the free acid; 10 _cc._ of the
carefully filtered liquor is placed in a beaker, and clear lime-water
is run in from a burette till permanent cloudiness is produced. The
quantity of lime-water employed is that which the acid is capable of
neutralising, without producing discoloration of the leather, and care
must be taken that the lime introduced with the butts does not exceed
this proportion. The explanation of the reaction is that dark-coloured
tannates of lime are formed, which are dissolved by the free acid so
long as it remains in excess. It must be remembered that this process
estimates all acids capable of retaining tannates of lime in solution,
including some so feeble as to have practically no plumping effect.
A liquor may have acidity equal to several _cc._ of lime-water, and
yet react absolutely alkaline to methyl-orange (see p. 9), a colour
which is distinctly reddened by small excess of acids, even so weak
as gallic, which is barely acid to the taste. Hence, the acidity of
a liquor available for plumping may be taken as represented by the
lime-water required to change the red of methyl-orange to yellow,
and if the liquor does not redden methyl-orange it is incapable
of plumping. If 5 or 10 drops of orange solution be added to the
pale filtered liquor from suspenders, there is no difficulty in
approximately hitting the point of change, but great accuracy is
not to be expected. If the liquor will not filter clear, kaolin (see
p. 119) may be used to clear it. It is well to test the lime-water
occasionally on 10 _cc._ of decinormal sulphuric or oxalic acid (p.
96), to make certain of its constancy. Lime-water should be kept in a
bottle with excess of lime, shaken occasionally, and a small quantity
filtered off as required. Liquors are frequently miscalled "sour" which
are not acid, but putrid. Such liquors will not plump, but reduce and
soften hides placed in them. (Compare also p. 185). Suspender liquors
usually consist mainly of liquors from the handler shift. If liquors be
used direct from the leaches, they generally produce harsh grain and
bad colour.
From the suspenders, the butts are transferred to the "handlers,"
where they are laid flat in the liquor. They are usually pulled over
by hooks, which are very apt to scratch the grain. Sometimes strings
are used, attached to the corners and held in notches or on pegs at the
edge of the pit. Other tanners place a frame below the pack, with ropes
at the four corners, by which it is raised sufficiently for the men to
grasp the top butts with their hands. This is only practicable in pits
of ample size. In American yards, the handling is almost universally
performed by tying the sides with strings or fastening them in a long
band by drawing the slit tail of one side through a hole in the nose
of the next, and inserting a wooden "key." The string of the sides is
then wound from one pit to another over a skeleton reel (Fig. 41).
This method is also used in the lime-yard, and is frequently employed
in England to handle offal, but it is not well adapted for butts. Fig.
42 shows the application of mechanical power in a Chicago yard for the
same purpose, by means of Ewart's drive-chain, which is manufactured in
this country by Ley's Malleable Castings Co., at Derby, to whom I am
indebted for the block.
[Illustration: Fig. 41.]
The handlers are generally worked in sets, to each of which a fresh
liquor is daily run, and the most forward pack is pulled over into it,
and is often also dusted down with a little fine bark or myrabolans.
The second pack follows into the liquor out of which the first has
been taken; the third into that of the second, and so on. Frequently
the greenest packs are handled up a second time in the course of the
day, and put down again in the same liquor. The strength of liquors,
and the length of time for which butts are retained in the handlers,
are varied; but a time of 1-2 months, and liquors of 20°-35° Bark.
are usual. It is well to divide the handlers into at least two sets.
Gambier is very useful, especially to the greener goods, and if
hemlock and other extracts are employed, their appropriate place is
in the forward handlers or earlier layers. New valonia liquors must
be avoided, but old layer liquors of considerable strength (up to 40°
Bark. where the handling is long continued) may be employed.
[Illustration: Fig. 42.]
At the end of this period, the butts are taken to the "layers" or
"bloomers," in which they are laid down with stronger liquors and much
larger quantities of "dust"; the latter is usually bark or valonia,
though mimosa is occasionally used. The liquors vary from 40° to 60°
or 70° Bark. in strength in mixed tannage, and the duration of each
layer from 10 days in the earlier stages to a month in the later ones.
For the best heavy tannages, 6-8 layers are required. Each time the
butts are raised, they should be mopped on the grain, to remove dirt
and loose bloom. Strong valonia liquors, or heavy valonia dusting,
causes a brown sandy crust to form on the freely exposed parts of the
butts. This is removed in striking, but is sometimes very troublesome
on rough dried dressing leather. In pure bark tannage, which, however,
is gradually becoming extinct, the liquors used are of necessity much
weaker, as it is extremely difficult to obtain liquors of more than
25°-30° Bark. from this material. The last layer, however, should
always have liquors of the greatest strength which can possibly be
obtained, or the leather will be deficient in firmness.
After receiving their last layer, the butts are well mopped or brushed
and washed up in a clear liquor, and thrown over a horse to drain
before going into the shed. In America, the Howard scrubber (Fig. 43)
is generally employed instead of hand labour at this stage. It consists
of 2 rotating wooden frames at the top of a pit, provided with brushes
or birch-brooms, and, when in use, enclosed by a cover A, through a
slit G in which the sides are inserted and drawn back, while water is
supplied by the pump B. Sometimes the brush-drums are placed one above
another, and the leather is passed in at the side.
[Illustration: PL. VI.
_E & F. N. Spon, London & New York._
"INK PHOTO" SPRAGUE & CO. LONDON
OPENING, STRETCHING AND STORING WHITE SKINS.]
[Illustration: Fig. 43.]
In mixed tannages, where the colour is dark, the leather is frequently
handled or suspended in a warm sumach or myrobalanes liquor, and
occasionally in dilute sulphuric or oxalic acids. If these acids are
not effectually removed before drying, the toughness of the leather
will be destroyed, and in extreme cases the leather will become brittle
and refuse to take black. In any case, strong acids are prejudicial to
the durability of the leather. In America, alternate baths of vitriol
and sugar of lead are frequently used for bleaching and weighting the
leather, but the colour given is not durable.
The great point to aim at, in arranging the mode of work of a tannery,
is to contrive that butts should always receive the strongest liquors
they can bear with safety, and that the strength should constantly
increase in a regular and systematic way. To attain this end, very
frequent handling and change of liquor are requisite in the early
stages, when the butts rapidly absorb the tannin presented to them. As
the process advances, the exterior part of the butt becomes thoroughly
tanned, and the liquor only slowly reaches the interior, which is yet
susceptible of its action, and hence longer layers in stronger liquors
are permissible.
The varied requirements of the trade render it difficult to give any
practical information as to the selection of tanning materials. As
a general rule, it is important at the outset to give the required
colour; and if materials undesirable in this respect are to be used for
the sake of cheapness, they should be introduced in the form of liquors
in the middle stages of the process, i. e. in the later handlers or
earlier layers. Materials used as dust generally have more effect in
producing bloom and colouring the leather, than those used in liquors
at this stage. Some information as to the respective qualities of the
different tanning materials will be found in the chapter on Tannins;
but even practical men are very deficient in accurate information on
these points, since many materials are never used alone, but invariably
in connection with others which mask their effects.
The use of extracts, and the demand for low-priced leathers, to
compete with the American tannages, has introduced still more rapid
methods than those described, and very fair-looking heavy leather has
been tanned in 5-10 weeks. These tannages are very various, but their
main feature is the free use of hot liquors, composed principally of
extracts and gambier. This treatment imparts great firmness, or more
properly speaking, hardness; but the leather is deficient in toughness,
and the grain usually cracks on bending sharply. Extract properly used
is, however, capable of making excellent leather; it is employed in at
least one of the highest priced tannages in the country.
It may be noted here, that when Continental writers speak of extracts
and extract tannage, what we should call liquor tannage only is meant,
and not specially the use of the concentrated extracts, to which alone
in England the term is applied.